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    <H1 CLASS="title">SeekableFilter</H1>
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<H2>Definition</H2>

<P>
    A SeekableFilter is a <A HREF="filter.html">Filter</A> whose <A HREF="../guide/modes.html">mode</A> refines <A HREF="../guide/modes.html#seekable">seekable</A>. 
</P>

<H2>Description</H2>

<P>
    A SeekableFilter operates on the character sequence controlled by a <A HREF="seekable_device.html">SeekableDevice</A>, providing access to a filtered sequence having the same character type. It may expose the filtered sequence in two ways:
    <OL>
        <LI STYLE="list-style-type:lower-roman">
            by defining member function <CODE>get</CODE>, <CODE>put</CODE> and <CODE>seek</CODE>.
        </LI>
        <LI STYLE="list-style-type:lower-roman">
            by defining member functions <CODE>read</CODE>, <CODE>write</CODE> and <CODE>seek</CODE>.
        </LI>
    </OL>
    The second alternative is provided for enhanced performance. SeekableFilters implementing this alternative are referred to as <I>Multi-Character</I>. (<I>See</I> <A HREF="multi_character.html">Multi-Character Filter</A>.)
</P>

<H2>Refinement of</H2>

<P><A HREF="input_filter.html">InputFilter</A>, <A HREF="output_filter.html">OutputFilter</A>.</P>

<H2>Associated Types</H2>

<TABLE CELLPADDING="5" BORDER="1">
    <TR><TD>Character type</TD><TD>The type of the characters in the filtered sequences</TD></TR>
    <TR>
        <TD>Category</TD>
        <TD>
            A type convertible to <A HREF="../guide/traits.html#category_tags"><CODE>filter_tag</CODE></A> and to <A HREF="../guide/modes.html#seekable"><CODE>seekable</CODE></A>
        </TD>
    </TR>
    <TR>
        <TD>Mode</TD>
        <TD>
            The unique <I>most-derived</I> <A HREF="../guide/modes.html#mode_tags">mode tag</A> to which Category is convertible
        </TD>
    </TR>
</TABLE>

<H2>Notation</H2>

<TABLE CELLPADDING="2">
    <TR><TD><CODE>F</CODE></TD><TD>- A type which is a model of SeekableFilter</TD></TR>
    <TR><TD><CODE>D</CODE></TD><TD>- A type which is a model of <A HREF="device.html">Device</A>, with the same character type as <CODE>F</CODE> and with mode convertible to the mode of <CODE>F</CODE></TD></TR>
    <TR><TD><CODE>Ch</CODE></TD><TD>- The character type of <CODE>F</CODE></TD></TR>
    <TR><TD><CODE>Tr</CODE></A></TD><TD>- <A HREF="../classes/char_traits.html"><CODE>boost::iostreams::char_traits&lt;Ch&gt;</CODE></A></TD></TR>
    <TR><TD><CODE>f</CODE></TD><TD>- Object of type <CODE>F</CODE></TD></TR>
    <TR><TD><CODE>d</CODE></TD><TD>- Object of type <CODE>D</CODE></TD></TR>
    <TR><TD><CODE>c</CODE></TD><TD>- Object of type <CODE>Ch</CODE></TD></TR>
    <TR><TD><CODE>s1</CODE></TD><TD>- Object of type <CODE>Ch*</CODE></TD></TR>
    <TR><TD><CODE>s2</CODE></TD><TD>- Object of type <CODE>const Ch*</CODE></TD></TR>
    <TR><TD><CODE>n</CODE></TD><TD>- Object of type <CODE>std::streamsize</CODE></TD></TR>
    <TR><TD><CODE>off</CODE></TD><TD>- Object of type <A HREF="../functions/positioning.html#synopsis"><CODE>stream_offset</CODE></A></TD></TR>
    <TR><TD><CODE>way</CODE></TD><TD>- Object of type <CODE>std::ios_base::seekdir</CODE></TD></TR>
    <TR><TD><CODE>which</CODE></TD><TD>- Object of type <CODE>std::ios_base::openmode</CODE></TD></TR>
    <TR><TD><CODE>io</CODE></TD><TD>- Alias for namespace <CODE>boost::iostreams</CODE></TD></TR>
</TABLE>

<H2>Valid Expressions / Semantics</H2>

<TABLE CELLPADDING="5" BORDER="1">
    <TR><TH>Expression</TH><TH>Expression Type</TH><TH>Category Precondition</TH><TH>Semantics</TH></TR>
    <TR>
        <TD>
            <PRE CLASS="plain_code"><CODE>typename <A HREF="../guide/traits.html#char_type_of_ref">char_type_of</A>&lt;F&gt;::type</CODE></PRE>
        </TD>
        <TD><CODE>typename</CODE> of the character type</TD>
        <TD ALIGN="center">-</TD><TD ALIGN="center">-</TD>
    </TR>
    <TR>
        <TD>
            <PRE CLASS="plain_code"><CODE>typename <A HREF="../guide/traits.html#category_ref">category_of</A>&lt;F&gt;::type</CODE></PRE>
        </TD>
        <TD><CODE>typename</CODE> of the category</TD>
        <TD ALIGN="center">-</TD><TD ALIGN="center">-</TD>
    </TR>
    <TR>
        <TD><PRE CLASS="plain_code"><CODE>f.get(d)</CODE></PRE></TD>
        <TD><CODE>Tr::int_type</CODE></TD>
        <TD ROWSPAN="2">
            Not convertible to <A HREF="../guide/traits.html#category_tags"><CODE>multi_char_tag</CODE></A>
        </TD>
        <TD>
            Returns the next character in the sequence controlled by <CODE>f</CODE>, or <CODE>Tr::eof()</CODE> if the end of the sequence has been reached. The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/read.html"><CODE>io::read</CODE></A> and <A HREF="../functions/putback.html"><CODE>io::putback</CODE></A>.

            <!-- reads up to <CODE>n</CODE> characters from the sequence controlled by <CODE>dev</CODE> into <CODE>s</CODE>, returning the number of characters read; returning a value less than n indicates end-of-sequence -->
        </TD>
    </TR>
    <TR>
        <TD><PRE CLASS="plain_code"><CODE>f.put(d, c)</CODE></PRE></TD>
        <TD><CODE>bool</CODE></TD>
        <TD>
            Writes the character <CODE>c</CODE> to the sequence controlled by <CODE>f</CODE>. The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/write.html"><CODE>io::write</CODE></A>.
        </TD>
    </TR>
    <TR>
        <TD><PRE CLASS="plain_code"><CODE>f.read(d, s1, n)</CODE></PRE></TD>
        <TD><PRE CLASS="plain_code"><CODE>std::streamsize</CODE></PRE></TD>
        <TD ROWSPAN="2">
            Convertible to <A HREF="../guide/traits.html#category_tags"><CODE>multi_char_tag</CODE></A>
        </TD>
        <TD>
            Reads up to <CODE></CODE> characters from the sequence controlled by <CODE>f</CODE> into the buffer <CODE>s1</CODE>, returning the number of characters read. Returning a value less than <CODE>n</CODE> indicates end-of-sequence. The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/read.html"><CODE>io::read</CODE></A> and <A HREF="../functions/putback.html"><CODE>io::putback</CODE></A>.
        </TD>
    </TR>
    <TR>
        <TD><PRE CLASS="plain_code"><CODE>f.write(d, s2, n)</CODE></PRE></TD>
        <TD><PRE CLASS="plain_code"><CODE>std::streamsize</CODE></PRE></TD>
        <TD>
            Writes n characters from the buffer <CODE>s2</CODE> to the sequence controlled by <CODE>f</CODE>. The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/write.html"><CODE>io::write</CODE></A>.
        </TD>
    </TR>
    <TR>
        <TD><PRE CLASS="plain_code"><CODE>f.seek(d, off, way)</CODE></PRE></TD>
        <TD><CODE>std::streampos</CODE></TD>
        <TD ALIGN="center">-</TD>
        <TD>
            <P CLASS="concept">
                Advances the read/write head by <CODE>off</CODE> characters, returning the new position, where the offset is calculated from:
            </P>
            <UL STYLE="margin:0,0,0,auto">
                <LI STYLE="list-style-type:disc;list-style-image:none">the start of the sequence if <CODE>way</CODE> is <CODE>ios_base::beg</CODE>
                <LI STYLE="list-style-type:disc;list-style-image:none">the current position if <CODE>way</CODE> is <CODE>ios_base::cur</CODE>
                <LI STYLE="list-style-type:disc;list-style-image:none">the end of the sequence if <CODE>way</CODE> is <CODE>ios_base::end</CODE>
            </UL>
            <P CLASS="concept">
                The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/read.html"><CODE>io::seek</CODE></A> and using <A HREF="../functions/putback.html"><CODE>io::write</CODE></A> if the <A HREF="../guide/modes.html">mode</A> of <CODE>F</CODE> is convertible to <A HREF="../guide/modes.html#output"><CODE>output</CODE></A>.
            </P>
        </TD>
    </TR>
</TABLE>

<H2>Exceptions</H2>

<P>
    Errors which occur during the execution of <CODE>get</CODE>, <CODE>put</CODE>, <CODE>read</CODE>, <CODE>write</CODE> or <CODE>seek</CODE> are indicated by throwing exceptions. Reaching the end of the sequence is not an error, but attempting to write past the end of the sequence is.
</P>

<P>
    After an exception is thrown, a SeekableFilter must be in a consistent state; further i/o operations may throw exceptions but must have well-defined behaviour. Furthermore, unless it is <A HREF="closable.html">Closable</A>, it must be ready to begin processing a new character sequence.
</P>

<H2>Models</H2>

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<P CLASS="copyright">&copy; Copyright 2008 <a href="http://www.coderage.com/" target="_top">CodeRage, LLC</a><br/>&copy; Copyright 2004-2007 <a href="https://www.boost.org/users/people/jonathan_turkanis.html" target="_top">Jonathan Turkanis</a></P>
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    Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at <A HREF="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>)
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